3.1 What is radiative forcing?

Radiative forcing (RF) is
an imbalance between the energy received by the earth and the
energy that is radiated back to space. It is usually expressed
as an amount of energy per surface area, in watts per square
meter (W/m2). A positive forcing represents a
situation where there is more energy coming in than there is
going out, which leads to a warming of the system, and negative
forcing leads to cooling.

Human activities have changed and continue to change the
Earth’s surface and
atmospheric composition.
Some of these changes have a direct or indirect impact on the
energy balance of the Earth and are thus
drivers of
radiative forcing.
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3.2 What are the current natural drivers of climate change?

Over the period of the industrial era, since 1750,
solar and volcanic forcings are the two
dominant natural contributors to global
climate change.

Solar forcing: Satellite observations of
total solar irradiance (TSI) changes since 1978 show
quasi-periodic cyclical variation with a period of roughly 11
years. Longer-term forcing can be estimated by comparing solar
minima (during which variability is least). This gives a
slightly negative forcing change between the 2008 minimum and
the 1986 minimum, and a slightly positive forcing since 1750.
There is a high confidence that solar forcing is much
smaller than the forcing caused by greenhouse gasses even if
current abilities to project solar irradiance are extremely
limited so that there is very low confidence concerning
future solar forcing.

It has been hypothesized that changes in the cosmic rays that
are associated to changes in solar activity affect climate
through changes in clouds dynamics. While cosmic rays do enhance
aerosolnucleation and may affect
cloud condensation, the effect is considered too small to have a
climate influence over the course of a solar cycle.

Volcanic
aerosols: The
forcing due to
stratospheric volcanic
aerosols is now well understood and there is a large negative
forcing for a few years after major volcanic eruptions. For
instance, the eruption of Mount Pinatubo, in 1991, caused a
one-year negative forcing of about –3.7 W/m2.
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3.3 What are the man-made drivers of climate change?

3.3.1
Human activity leads to change in the
atmospheric composition
either directly (via emissions of gases or
particles) or
indirectly (via atmospheric chemistry).

Anthropogenic emissions
have driven the changes in
greenhouse gas
concentrations during the industrial era (so since 1750). As the
historical evolution of the concentrations of these gases is
well known, and since their greenhouse properties are also well
known and defined, calculating the
radiative forcing (RF)
due to greenhouse gases
gives well defined values with a very high confidence.
(Figure TS.6). Based on concentration changes, the RF of all
greenhouse gases that are well-mixed into the
atmosphere was in
2011 at 2.83 W/m2 [2.54 to 3.12]. Over the last 15
years, CO2 has been the dominant contributor to the
increase in radiative forcing from greenhouse gases while
methane and
nitrogen oxide also
are important contributors. Halocarbons, such as
chloro-fluoro-carbons (CFCs) used in the past in cooling
systems, among other things, are very powerful greenhouse gases,
and despite being present in relatively very small amounts
compared to CO2, also contribute to radiative
forcing. The growth of forcing from all greenhouse gases is
slower now than it was in the 1970s and 1980s because emissions
from greenhouse gasses other than CO2 have been
increasing more slowly5

Some greenhouse gases,
such as ozone and water
vapor 6, also contribute to
anthropogenic forcing.
In the lower atmosphere,
ozone leads to a positive forcing, while in the upper
atmosphere, the depletion of the
ozone layer, induced in
particular by chlorineatoms radicals produced
from the decomposition of halomethanes, has led to negative
forcing. Ozone is not emitted directly into the atmosphere;
instead it is formed in the high atmosphere when oxygen reacts
with ultraviolet light, and in the lower atmosphere when
nitrogen oxide or
hydrocarbons react with light. There is strong evidence that
ozone in the lower atmosphere affects plants, and reduces their
CO2 uptake, in turn contributing to the increase
of CO2 in the atmosphere.

The impact of greenhouse gasses can be estimated in a number
of ways, but the two main ways are in terms of Global Warming
Potential (GWP) or in terms of Global Temperature change
Potential (GTP). GWP expresses the
radiative forcing
brought by a particular greenhouse
gas in reference to the warming potential of
carbon dioxide,
whereas GTP expresses the temperature change brought by this
same gas, and it takes into account the response of the climate
system.
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5 In particular halocarbon gases production and
uses have been regulated and some like CFCs progressively banned.

6 It should be noted that the main greenhouse gas
present in the atmosphere is water vapor but its contribution to
radiative forcing is considered constant

3.3.2 Aerosols are tiny liquid
droplets or particles (such
as dust from volcanos) that are in suspension in the
atmosphere. They
contribute to the energy balance by interacting with clouds
formation, or directly by reflecting or absorbing light. There
has been progress in the last few years on the understanding of
the properties and distribution of aerosols but significant
uncertainties remain due to difficulty of observation and their
large variability. The overall
radiative forcing
produced by aerosols is negative (so they cause a cooling of the
atmosphere), but with a large range of uncertainty.

The forcing from black carbon
particles (BC) on snow
and ice is assessed but with a low confidence to be slightly
positive. It represents a global
mean surface
temperature change per unit forcing larger than from
CO2 primarily because all of the energy is deposited
directly into the cryosphere. Ice
sheets, glaciers
and sea ice reflects solar
energy back into space (this reflectivity of the earth surface
is called “albedo”) , and the dark deposition on ice has a feedback impact on climate that can be significant in the polar and other snow or ice covered regions.

Despite the large uncertainty ranges on the importance of
aerosol forcing,
there is a high confidence that
aerosols have offset a
substantial portion of the forcing due to
greenhouse gases.
Aerosol-cloud interactions can also influence the character of
individual storms, but evidence for a systematic aerosol effect
on storm or precipitation intensity is more limited and
ambiguous.
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3.3.3
There is robust evidence that
anthropogenicland use changes, such as
deforestation, have
increased the land surface albedo ( which is different in a
darker green forest than in a paler field, for instance). There
are still uncertainties in the evaluation of the albedo
of natural and managed surfaces (such as croplands, pastures)
and the influence of the changes in land use over the last
centuries is still debated. Changes in land use also causes
other modifications such as changes in surface roughness and in
river runoff that also have an impact on local temperatures and
are difficult to quantify.

Persistent contrails from aviation is another contribution to
a positive radiative
forcing. This forcing can be much larger regionally but
it does not seem to produce observable regional effects on
either the mean or diurnal
range of surface temperature.
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3.4 What in the role of feedback mechanisms in climate change?

Feedback mechanisms also
play an important role in determining (future)
climate change.
Indeed, climate change may induce modification in natural cycles
which may reinforce or dampen the expected temperature increase.

For example:

Snow and ice albedo
feedbacks are
known to be positive: the warmer it gets the less snow there
is, the darker and hotter the ground is, the lower is the
reemission factors.

There can also be feedbacks in cloud
cover, although there are still large uncertainties
attached to their importance and influence.

The emissions of
methane
(CH4) by wetlands (associated to
anaerobic degradation of
organic matter)
will increase in a warming climate, but it is not clear if
the areas of wetlands will increase or decrease, so their
overall resulting impact is not clear.